Efficient optical interfacing of spin-bearing quantum emitters is a crucial ingredient for quantum networks. A promising route therefore is to incorporate host materials as minimally processed membranes into open-access microcavities: it enables significant emission enhancement and efficient photon collection, minimizes deteriorating influence on the quantum emitter, and allows for full spatial and spectral tunability. Here, we study the properties of a high-finesse fiber Fabry-Pérot microcavity with integrated single-crystal diamond membranes by scanning cavity microscopy. We observe the spatially resolved effects of the diamond-air interface on the cavity-mode structure: a strong correlation of the cavity finesse and mode structure with the diamond thickness and surface topography, prevalent transverse-mode mixing under diamondlike conditions, and mode-character-dependent polarization-mode splitting. Our results reveal the influence of the diamond surface on the achievable Purcell enhancement, which helps to clarify the route towards optimized spin-photon interfaces.

The Harrow-Hassidim-Lloyd quantum algorithm was proposed to solve linear systems of equations $A\overrightarrow{x}=\overrightarrow{b}$ and it is the core of various applications. However, there is no explicit quantum circuit for the subroutine that maps the inverse of the problem matrix $A$ into an ancillary qubit. This makes implementation in current quantum devices challenging, forcing us to use hybrid approaches. Here, we propose a systematic method to implement this subroutine, which can be adapted to other functions $f(A)$ of matrix $A$, we present a co-designed quantum processor that reduces the depth of the algorithm, and we introduce its digital-analog implementation. The depth of our proposal scales with precision $ϵ$ as $\mathcal{O}({ϵ}^{-1})$, which is bounded by the number of samples allowed for a certain experiment. The co-design of the Harrow-Hassidim-Lloyd algorithm leads to a “kitelike” architecture, which allows us to reduce the number of required swap gates. Finally, merging a co-design quantum processor architecture with a digital-analog implementation contributes to the reduction of noise sources during the experimental realization of the algorithm.

We introduce a hybrid tripartite quantum system for strong coupling between a semiconductor spin, a mechanical phonon, and a microwave excitation of a superconducting circuit. Consisting of a piezoelectric resonator with an integrated diamond strain concentrator, this system achieves microwave-acoustic and spin-acoustic coupling rates of approximately megahertz or greater, allowing simultaneous ultrahigh cooperativities (approximately ${10}^3$ and approximately ${10}^2$, respectively). From finite-element modeling and master-equation simulations, we estimate superconducting-circuit-to-spin quantum state transfer fidelities exceeding 0.95 on the basis of separately demonstrated device parameters. We anticipate that this device will enable hybrid quantum architectures that leverage the advantages of both superconducting circuits and solid-state spins for information processing, memory, and networking.

Paramagnetic defects and nuclear spins are often the major sources of decoherence and spin relaxation in solid-state qubits realized by optically addressable point defect spins in semiconductors. It is commonly accepted that a high degree of depletion of nuclear spins can enhance the coherence time by reducing magnetic noise. Here we show that the isotope purification beyond a certain optimal level can become contraproductive when both electron and nuclear spins are present in the vicinity of the qubits, particularly for half-spin systems. Using state-of-the-art numerical tools and considering the silicon-vacancy qubit in various spin environments, we demonstrate that the coupling of the spin-3/2 qubit to a spin bath of spin-1/2 point defects in the lattice can be significantly enhanced by isotope purification. The enhanced coupling shortens the spin-relaxation time that in turn may limit the coherence time of spin qubits. Our results can be generalized to triplet point defect qubits, such as the nitrogen-vacancy center in diamond and the divacancy in silicon carbide.

The development of modular and versatile quantum interconnect hardware is a key next step in the scaling of quantum information platforms to larger size and greater functionality. For superconducting quantum systems, fast and well-controlled tunable circuit couplers will be paramount for achieving high-fidelity and resource-efficient connectivity, whether for performing two-qubit gate operations, encoding or decoding a quantum data bus, or interfacing across modalities. Here we propose a versatile and internally tunable double-transmon coupler (DTC) architecture that implements tunable coupling via flux-controlled interference in a three-junction dc superconducting quantum interference device. Crucially, the DTC possesses an internally defined zero-coupling state that is independent of the coupled data qubits or circuit resonators. This makes it particularly attractive as a modular and versatile design element for realizing fast and robust linear coupling in several applications such as high-fidelity two-qubit gate operations, qubit readout, and quantum bus interfacing.

High-fidelity quantum gate operations are essential for achieving scalable quantum circuits. In spin qubit quantum computing systems, metallic gates and antennas that are necessary for qubit operation, initialization, and readout, also cause detrimental effects by enhancing fluctuations of electromagnetic fields. Therefore, evanescent wave Johnson noise (EWJN) caused by near-field thermal and vacuum fluctuations becomes an important unmitigated noise, which induces the decoherence of spin qubits and limits the quantum gate operation fidelity. Here, we first develop a macroscopic quantum electrodynamics theory of EWJN to account for the dynamics of two spin qubits interacting with metallic circuitry. Then we propose a numerical technique based on volume integral equations to quantify EWJN strength in the vicinity of nanofabricated metallic gates with arbitrary geometry. We study the limits to two-spin-qubit gate fidelity from EWJN-induced relaxation processes in two experimentally relevant quantum computing platforms: (a) the silicon quantum dot system and (b) nitrogen-vacancy centers in diamond. Finally, we introduce a Lindbladian engineering method to optimize the control pulse sequence design and show its enhanced performance over Hamiltonian engineering in mitigating the influence of thermal and vacuum fluctuations. Our work leverages advances in computational electromagnetics, fluctuational electrodynamics, and open quantum systems to suppress the effects of near-field thermal and vacuum fluctuations and reach the limits of two-spin-qubit gate fidelity.

We propose to use quantum computers to simulate infection spreading in networks. We first show the analogy between the infection distribution and spin-lattice configurations with Ising-type interactions. Then, since the spreading process can be modeled as a classical Markovian process, we show that the spreading process can be simulated using the evolution of a quantum thermal dynamic model with a parameterized Hamiltonian. In particular, we analytically and numerically analyze the evolution behavior of the Hamiltonian, and prove that the evolution simulates a classical Markovian process, which describes the well-known epidemiological stochastic susceptible and infectious (SI) model. A practical method to determine the parameters of the thermal dynamic Hamiltonian from epidemiological inputs is exhibited. As an example, we simulate the infection spreading process of the SARS-Cov-2 variant Omicron in a small-world network.

Realizing a quantum network will require long-lived quantum memories with optical interfaces incorporated into a scalable architecture. Color-center quantum emitters in diamond have emerged as a promising memory modality due to their optical properties and compatibility with scalable integration. However, developing a scalable color-center emitter module requires significant advances in the areas of heterogeneous integration and cryogenically compatible packaging. Here we report on a cryogenically stable and network compatible quantum emitter module for memory use. This quantum emitter module is a significant development towards advanced quantum networking applications such as distributed sensing and processing.

Quantum optical microcombs in integrated ring resonators generate entangled photon pairs over many spectral modes, and allow the preparation of high-dimensional qudit states. Ideally, those sources should be programmable and have a high generation rate, with comb lines tightly spaced for the implementation of efficient qudit gates based on electro-optic frequency mixing. While these requirements cannot all be satisfied by a single resonator device, for which there is a trade-off between the high generation rate and tight bin spacing, a promising strategy is the use of multiple resonators, each generating photon pairs in specific frequency bins via spontaneous four-wave mixing. Based on this approach we present a programmable silicon photonics device for the generation of frequency-bin-entangled qudits, in which bin spacing, qudit dimension, and the bipartite quantum state can be reconfigured on chip. Using resonators with a radius of $22\text{μ}\mathrm{m}$, we achieve a high brightness [about $\mathrm{MHz}/(\mathrm{mW}{)}^2$] per comb line with a bin spacing of 15 GHz, and fidelities above 85% with maximally entangled Bell states up to a Hilbert space dimension of 16. By individually addressing each spectral mode, we realize states that cannot be generated on chip using a single resonator. We measure the correlation matrices of maximally entangled two-qubit and two-qutrit states on a set of mutually unbiased bases, finding fidelities exceeding 98%, and indicating that the source can find application in high-dimensional secure communication protocols.

We propose a new phase detection technique based on a flux-switchable superconducting circuit, the Josephson digital phase detector (JDPD), which is capable of discriminating between two phase values of a coherent input tone. When properly excited by an external flux, the JDPD is able to switch from a single-minimum to a double-minima potential and, consequently, relax in one of the two stable configurations depending on the phase sign of the input tone. The result of this operation is digitally encoded in the occupation probability of a phase particle in either of the two JDPD wells. In this work, we demonstrate the working principle of the JDPD up to a frequency of 400 MHz with a remarkable agreement with theoretical expectations. As a future scenario, we discuss the implementation of this technique to superconducting qubit readout. We also examine the JDPD compatibility with the single-flux-quantum architecture, employed to fast-drive and measure the device state.

Processing quantum information using quantum three-level systems or qutrits as the fundamental unit is an alternative to contemporary qubit-based architectures with the potential to provide significant computational advantages. We demonstrate a fully programmable two-qutrit quantum processor by utilizing the third energy eigenstates of two transmons. We develop a parametric coupler to achieve excellent connectivity in the nine-dimensional Hilbert space enabling efficient implementations of two-qutrit gates. We characterize our processor by realizing several algorithms like Deutsch-Jozsa, Bernstein-Vazirani, and Grover’s search. Our hardware-efficient protocols allow us to show that two stages of Grover’s amplification can improve the success rates of an unstructured search with quantum advantage. Our results pave the way for building fully programmable ternary quantum processors using transmons as building blocks for a universal quantum computer.

Twin-field quantum key distribution (TFQKD) enables two distant parties to establish a shared secret key, by interfering weak coherent pulses (WCPs) in an intermediate measuring station. This allows TFQKD to reach greater distances than traditional QKD schemes and makes it the only scheme capable of beating the repeaterless bound on the bipartite private capacity. Here, we generalize TFQKD to the multipartite scenario. Specifically, we propose a practical conference key agreement protocol that only uses WCPs and linear optics and prove its security with a multiparty decoy-state method. Our protocol allows an arbitrary number of parties to establish a secret conference key by single-photon interference, enabling it to overcome recent bounds on the rate at which conference keys can be established in quantum networks without a repeater.

Preshared entanglement can significantly boost communication rates in the high thermal-noise and low-brightness transmitter regime. In this regime, for a lossy bosonic channel with additive thermal noise, the ratio between the entanglement-assisted capacity and the Holevo capacity—the maximum reliable communication rate permitted by quantum mechanics without any preshared entanglement—scales as $\log (1/{\overline{N}}_S)$, where the mean transmitted photon number per mode, ${\overline{N}}_S\ll 1$. Thus, preshared entanglement, e.g., distributed by the quantum internet or a satellite-assisted quantum link, promises to significantly improve low-power radio-frequency communications. In this paper, we propose a pair of structured quantum transceiver designs that leverage continuous-variable preshared entanglement (generated, e.g., from a down-conversion source), binary phase modulation, and non-Gaussian joint detection over a codeword block, to achieve this scaling law of capacity enhancement. Furthermore, we describe a modification to the aforesaid receiver using a frontend that uses sum-frequency generation sandwiched with dynamically programmable in-line two-mode squeezers, and a receiver backend that takes full advantage of the output of the receiver’s frontend by employing a nondestructive multimode vacuum-or-not measurement to achieve the entanglement-assisted classical communication capacity.

On-chip mode division (de)multiplexing plays a significant role in integrated devices to greatly improve communication capacity. One of the goals of mode division (de)multiplexing is to realize devices with larger bandwidth and better fabrication tolerance. However, integrated broadband mode division (de)multiplexing devices have rarely been reported up to now. In this work, we experimentally realize a broadband mode division demultiplexing device with a high degree of fabrication tolerance based on a lithium niobate-on-insulator waveguide array. By taking advantage of the fact that different modes in the waveguide array have different group velocities, we experimentally confirm that different modes in the waveguide array have different transverse displacements. The implementation of the integrated broadband mode division demultiplexer may have future applications in information processing technology, quantum communication, and quantum computing.

Coupling of space-separated resonators is interesting for quantum and communication technologies. In this work, we show that antiferromagnetic resonance in separated parallel-plane slabs of hematite ($\alpha$-${\mathrm{Fe}}_2{\mathrm{O}}_3$) couple cooperatively to terahertz electromagnetic cavity modes formed by the slabs themselves. We demonstrate control of these hybridized magnon-polariton modes either by tuning the distance between the slabs in the range of up to a few millimters or by scanning their temperatures in a range above room temperature. Analysis of measured spectra with three distinct theoretical models shows that the best approximations can be obtained with a model based on classical electromagnetism. Cavity-mediated coupling allows the engineering of resonators at millimeter-range frequencies (millielectronvolts).

The side-channel-secure (SCS) quantum key distribution (QKD), SCS QKD has attracted much attention due to its higher security compared to the measurement-device-independent protocols. It is not only measurement-device-independent secure, but also secure against source-state side channels provided that Eve has no access to Alice’s or Bob’s setups inside their labs. In the original protocol for SCS QKD, the side channels and the intensities of the coherent sources are allowed to be different in different time windows, but the vacuum sources of Alice and Bob have to be perfect, which is impossible in practice due to the finite extinction ratio of the intensity modulators. In this paper, we present a method that does not need the perfect vacuum source requested by the original protocol and assures the result as secure as the original protocol. With the upper bounds of the amplitudes of the nonvacuum part of the sources, the secure key rate of our SCS QKD protocol here with imperfect vacuum and unstable sources can be calculated. The numerical results show that, the secure distance of the SCS QKD protocol exceeds 150 km, provided that the intensity of the imperfect vacuum source is less than ${10}^{-8}$, which can be achieved in the experiment by a two-stage intensity modulator, where we also show that the active odd-parity pairing method and standard pairing method using two-way classical communication can be applied to the SCS QKD protocol to improve the key rate. Given the side-channel security based on the existing technological level, this work makes it possible to realize side-channel-secure QKD with real devices.

Banks are required to set aside funds in their income statement, known as a loan-loss provision (LLP), to account for potential loan defaults and expenses. By treating the LLP as a global constraint, we propose a hybrid quantum-classical algorithm to solve a specific case of quadratic constrained binary optimization (QCBO) models for loan-collection optimization. The objective is to find a set of optimal loan-collection actions that maximizes the expected net profit presented to the bank as well as the financial welfare in the financial network of loanees, while keeping the LLP at its minimum. Our algorithm consists of three parts: a classical divide-and-conquer algorithm to enable a large-scale optimization, a quantum alternating operator ansatz (QAOA) algorithm to maximize the objective function, and a classical sampling algorithm to handle the LLP. We apply the algorithm to a real-world data set with 600 loanees and five possible collection actions. The QAOA is performed using up to 35 qubits on a classical computer. We show that incorporating the QAOA can enhance the expected net profit by approximately $70\mathrm{\%}$ in comparison to scenarios where the QAOA is absent from the hybrid algorithm. Although this improvement does not constitute definitive evidence of quantum advantage, our work illustrates the use of near-term quantum devices to tackle real-world optimization problems.

Non-Gaussian modulation can improve the performance of continuous-variable quantum key distribution (CV QKD). For Gaussian-modulated coherent-state CV QKD, photon subtraction can realize non-Gaussian modulation, which can be equivalently implemented by non-Gaussian postselection. However, non-Gaussian reconciliation has not been deeply researched, which is one of the key technologies in CV QKD. In this paper, we propose a non-Gaussian reconciliation method to obtain identical keys from non-Gaussian data. Multidimensional reconciliation and multiedge-type low-density parity-check codes (MET LDPC) are used in a non-Gaussian reconciliation scheme, where the layered belief propagation decoding algorithm of MET LDPC codes is used to reduce the decoding complexity. Furthermore, we compare the error-correction performance of Gaussian data and non-Gaussian data. The results show that the error-correction performance of non-Gaussian data is better than Gaussian data, where the frame error rate can be reduced by 50 % for code rate 0.1 at SNR of 0.1554 and the average iteration number can be reduced by 25 %.